Retracted monolayers

Chapman J A and Tabor D 1957 An electron diffraction study of retracted monolayers Proc. R. See. A 242 96-107... 

Shafrin, E.G. and Zisman, W.A. "Constitutive Relations in the Wetting of Low-Energy Surfaces and the Theory of the Retraction Method of Preparing Monolayers," Naval Research Labs Report 5394, Surface Chemistry Branch, Chemistry Division, October 21, 1959. 

In the first set of measurements the rate of carbon build-up on a Ni(lOO) surface was measured at various temperatures as follows (1) surface cleanliness was established by AES (2) the sample was retracted into the reaction chamber and exposed to several torr of CO for various times at a given temperature (3) after evacuation the sample was transferred to the analysis chamber and (4) the AES spectra of C and Ni were measured. Two features of this study are noteworthy. First, two kinds of carbon forms are evident - a carbidic type which occurs at temperatures < 650 K and a graphite type at temperatures > 650 K. The carbide form saturates at 0.5 monolayers. Second, the carbon formation data from CO disproportionation indicates a rate equivalent to that observed for methane formation in a H2/CO mixture. Therefore, the surface carbon route to product is sufficiently rapid to account for methane production with the assumption that kinetic limitations are not imposed by the hydrogenation of this surface carbon. 

The final state of the interface is now just unfavaourable for spreading. This causes the initial spreading to be stopped, and can even result in the film retracting slightly to form very flat lenses, the rest of the water surface being covered by a monolayer of benzene. 

Shafrin, E.G. and Zisman, W.A. (1960) Constitutive relations in the wetting of low-energy surfaces and the theory of the retraction method of preparing monolayers. /. Phys. Chem., 64, 519-24. 

A bare platinum electrode was coated with a close-packed monolayer of n-octadecylamine by retraction from the molten crystal compound (2). The contact angle of methylene iodide with the monolayer was 68° to 70°, which should be true if the film was close packed. This film-coated electrode was used with the FEP resin-covered gold electrode in the experimental sequence described... 

Our own investigations have concerned (a) liquid spreading on solids and the laws relating the equilibrium contact angle and the critical surface tension of wetting to solid and liquid constitution (26, 27, 28, 53, 54,62), (b) liquid/liquid displacement from solid surfaces (1,5), (c) the properties of adsorbed monolayers on solids and their relation to the monolayer retraction method (28, 54, 62), (d) the surface electrostatic potentials of adsorbed organic monolayers on metals (9, 10, 11, 58, 59), (e) the effects of surface constitution on adhesion and abhesion (60),... 

Adsorption experiments were conducted on chromium, platinum, cadmium, and zinc the sources and preparation of these metal specimens have been reported previously (16). In preparing adsorbed, mono-molecular layers by adsorption directly from the molten pure acid (5), the clean adsorbing substrate was first heated to a temperature just above the melting point of the acid (see Table I), a few crystals of the acid were sprinkled on the surface, and the resulting pool of molten acid was teased over the whole surface with a previously freshly flamed platinum wire. If spontaneous retraction of the liquid acid did not occur, the specimen was allowed to cool and all of the solidified material adhering on top of the adsorbed monolayer was removed by appropriate solvent treatments as discussed below. 

Figure 12.12 (a) Idealised single-molecule transport experiment, (b) Single-molecule transport experiment with optimal structural control based on a (1) epitaxial molecular monolayer with well-characterised adsorbate-substrate bond and (2) a specific, chemically well-defined tip-molecule contact, (c) Mechanically gated single-molecule wire based on experiment in b and STM tip retraction. 

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